First lady Michelle Obama on Thursday pilloried GOP presidential nominee Donald Trump for declaring he may not accept the results of November’s election if he loses. In the process, she delivered yet another powerful argument in favor of Democratic presidential nominee Hillary Clinton, cementing her status as one of Clinton’s most effective advocates on the campaign trail.

“You do not keep American democracy ‘in suspense,’ because look, too many people have marched and protested, and fought and died for this democracy,” Obama said, referring to Trump’s astonishing statement from Wednesday night’s debate, when he refused to say definitively whether he would accept the election results.

The first lady was campaigning in Phoenix, where in recent weeks Clinton has narrowed Trump’s lead and turned the traditionally Republican state into a tossup.

Obama said Trump’s statement represented a threat to “the very idea of America itself,” and argued that his repeated claims that the election is “rigged” was an effort by his campaign to stoke voter apathy and keep voters away from the polls.

“For the record, our democracy is revered around the world, and free elections are the best way on Earth to choose our leaders. This is how we elected John F. Kennedy, Ronald Reagan, two George Bushes and Barack Obama. It has worked for decades,” she said. “We are fortunate, and I have traveled the world, we are fortunate to live in a country where the voters decide our elections, where the voters decide who wins or loses. Period. End of story. And when a presidential candidate threatens to ignore our voices and reject the outcome of the election, he is threatening the very idea of America itself.”

Without even having to mention Trump by name, Obama has repeatedly given some of the most stirring arguments for Clinton and against Trump, often using the businessman’s statements against him.

Campaigning in New Hampshire last week, Obama delivered a powerful denouncement of Trump’s history of demeaning women, which quickly went viral.

In her remarks on Thursday, Obama said her office was inundated with responses to the speech, in which she spoke in personal terms about how the 2005 video of Trump bragging about sexual assault had “shaken me to my core.”

Obama attacked Trump’s divisiveness, saying that his propensity to insult and denigrate groups, from immigrants to veterans, from women to the disabled, from Muslims to African Americans, stems from his background as a wealthy real estate mogul.

“Hillary’s opponent comes from a different place. I don’t know, perhaps living life high up in a tower, in a world of exclusive clubs, measuring success by wins and losses, the number of zeroes in your bank account, perhaps you just develop a different set of values,” she said of Trump. “Maybe with so little exposure to people who are different than you, it becomes easy to take advantage of those who are down on their luck, folks who play by the rules, pay what they owe. Because to you, those folks just aren’t very smart, and seem somehow less deserving. And if you think this way, then it’s easy to see this country as us versus them, and it’s easy to dehumanize ‘them,’ to treat ‘them’ with contempt, because you don’t know ‘them.’ You can’t even see ‘them.’”

Obama also praised Clinton’s policy experience and preparedness, which Trump has often used to attack her.

“Here’s the thing about Hillary: She is a policy wonk. And let me tell you, just for the record: When you are president, that is a good thing,” she said. “When you are president, knowing what you are doing is a good thing.”

“Hillary has comprehensive policies to help people. Her opponent has tweets,” she continued. “You decide.”